The present invention relates to a manufacturing method of ceramic circuit board used in various electronic appliances, and more particularly to a manufacturing method of ceramic circuit board for forming a conductor pattern on a ceramic substrate by intaglio printing.
Recently, electronic appliances are reduced progressively in size, and electronic components used in the electronic appliances are rapidly becoming smaller in size. This trend is the same in printed circuit boards and ceramic circuit boards for forming electronic circuits. As a result, fine forming techniques of conductors and via holes for composing circuits, multi-layer forming techniques, and wiring techniques of higher density are demanded. In addition to the trend of higher integration of LSI chips, the number of electrodes is increasing, the electrode pitches are becoming narrower, and a mounting method suited to multiple-pin and narrow-pitch trend is demanded.
Generally, the manufacturing method of this kind of ceramic circuit board has been as shown in FIG. 30.
That is, FIG. 30 is a process chart showing a conventional manufacturing method of ceramic circuit board, in which a green sheet 51 of about 0.2 mm in thickness mainly made of alumina is prepared (step a), and via holes 52 are drilled in necessary positions by punching or CO2 laser processing (step b). The via holes 52 are filled with conductive paste (for example, tungsten paste) by screen printing or other method, and dried, and vias 53 are formed (step c). A conductor pattern 54 of conductive paste is formed on the green sheet 51 in a desired circuit pattern by screen printing method (step d), and a first layer of circuit board 55 is formed.
Similarly, a second layer of circuit board 56, a third layer of circuit board 57, and, if necessary, a fourth layer of circuit board 58 are prepared (step e), and they are positioned and pressed, and a laminated circuit board 59 is obtained (step f). Finally, it is baked at high temperature of 900 to 1600xc2x0 C., so that a ceramic multi-layer substrate 60 is obtained (step g).
Thus, according to this method, a higher density is achieved by increasing the number of layers.
However, such a conventional manufacturing method of ceramic circuit board has the following problems.
1) Since the conductor pattern 54 is formed by screen printing, it is very difficult to define the line width (W)/line interval (spacing)=75 xcexcm/75 xcexcm or less.
2) In the case of screen printing for forming the conductor pattern 54, as the pattern is finer, the film thickness is smaller, and for example, at W=75 xcexcm, the film thickness is only about 5 xcexcm, and the wiring resistance is higher.
3) When tungsten is used as wiring material, in particular, the wiring resistance is about 3 to 5 times higher than in the wiring of silver (Ag) or copper (Cu), and this defect is more significant when the pattern is finer, and it cannot be used as electric component in those cases.
4) Since the substrate and wiring materials are baked simultaneously at high temperature of about 900 to 1600xc2x0 C., a material shrinkage of about 15 to 20% occurs after baking, and hence the substrate dimensions fluctuate widely. Because of shrinkage and fluctuation, the wiring dimensions also vary significantly, and connection with very fine bumps on the LSI is inaccurate in precision, and the mounting yield is lowered.
5) Further, when forming the conductor pattern 54 by screen printing, the line width is more than 75 xcexcm (generally, 120 xcexcm or more should be required in consideration of the printing yield and wiring resistance), and many lines are formed in a narrow area, and more layers for wiring are required, and as the number of layers is increased, or the dimensional precision is improved for the sake of mounting precision, the substrate cost becomes higher.
The invention is intended to solve such conventional problems, and it is hence an object thereof to realize a manufacturing method of ceramic circuit board as specified below.
1) To realize low wiring resistance, and very high wiring density, capable of forming conductor pattern in a very narrow line width, and forming conductor film in a large thickness in spite of narrow line width.
2) To form fine via patterns simultaneously when forming conductor pattern, and form dense wiring pattern of high dimensional precision.
3) To connect securely between layers if the number of layers is increased, and connect securely when mounting an LSI chip face down on the surface layer.
To achieve the object, the manufacturing method of ceramic circuit board of the invention is a manufacturing method of ceramic circuit board for forming a first conductor pattern on a ceramic substrate by intaglio printing, comprising:
(a) a step of preparing an intaglio by forming a first groove on the surface of a flexible resin base material in a pattern corresponding to a first conductor pattern, and forming a second groove in a pattern corresponding to the via or bump of the first conductor pattern, deeper than the first groove,
(b) a step of supplying a conductive paste to fill up the first and second grooves, and drying,
(c) a step of supplying an additional conductive paste to cover up for the volume loss by drying of the conductive paste at step (b), and filling up and drying again, repeating as many times as required,
(d) a step of adhering together the intaglio and a ceramic substrate by applying heat and pressure in a specified range, and
(e) a step of parting the intaglio from the ceramic substrate, transferring the pattern of conductive paste on the ceramic substrate, and baking to form a first conductor pattern, in which the conductive paste is supplied at steps (b) and (c) by using any one of screen mask, metal mask, and drawing device.
In this constitution, a fine wiring pattern of a sufficient film thickness can be easily formed, and a ceramic circuit board of low wiring resistance, high wiring density, and high dimensional precision of wiring pattern is obtained. Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the exemplary embodiments of the present invention. The invention itself can better be understood by reference to the following detailed description and the accompanying drawings.